Drive circuits for switched reluctance motors are comprised of power switching devices and diodes for sequentially energizing the motor phase windings, with or without overlap, in accordance with the position of the rotor to produce a rotating magnetic field that interacts with the rotor poles to produce torque of a desired direction and magnitude.
The standard drive circuit topology includes two power transistors and two power diodes per motor phase. FIG. 1 shows the standard drive circuit topology as applied to two phases of a polyphase machine. Power is supplied to the motor phase windings A and B from a DC source such as battery 10, the winding A being connected across battery 10 by transistors 12 and 14, and the winding B being connected across battery 10 by transistors 16 and 18. Free-wheeling diodes 20-26 circulate inductive current in the respective phase windings when one or both of the respective transistors are turned off. In operation, a given phase winding is energized by turning on the upper and lower transistors associated with that phase. For example, winding A is energized by turning on transistors 12 and 14. Current in the phase winding is regulated by pulse-width-modulating (PWM) either of the upper and lower transistors while the other transistor is maintained conductive, the winding current during off periods of the PWM being re-circulated through the conductive transistor and one of the free-wheeling diodes 20 or 22. When both upper and lower transistors are turned off to terminate the energization interval, both free-wheeling diodes 20 and 22 conduct to return the winding current to battery 10. This allows the winding voltage to rise slightly higher than the voltage of battery 10, and the current quickly collapses.
A number of alternate drive circuit topologies have been developed in an effort to reduce the number of power devices, and therefore, the cost of the drive circuit. A prior art circuit which halves the number of transistors and diodes is shown in FIG. 2. This drive circuit, sometimes referred to as a split-link circuit, reduces the number of power devices by using the capacitors C1 and C2 to establish an intermediate voltage link or bus 40, the phase windings being connected between the bus 40 and one of the positive or negative terminals of battery 10 by a respective transistor. In FIG. 2, phase winding A is connected between bus 40 and the positive battery terminal by transistor 42, while winding B is connected between bus 40 and the negative battery terminal by transistor 44. The phase winding A is connected in parallel with capacitor C1, and returns inductive energy at commutation to capacitor C2 via free-wheeling diode 46. Conversely, the phase winding D is connected in parallel with capacitor C2, and returns inductive energy at commutation to capacitor C1 via free-wheeling diode 48. One such circuit, described in the U.S. Pat. No. 4,835,408 to Ray et al., additionally includes circuitry for regulating the voltage on the intermediate bus 40 by suitably adjusting the conduction period of one or more of the phase windings. While such topologies are advantageous for their reduced number of power transistors and diodes per phase, only one-half of the supply voltage is available for energizing the phase windings. As a result, the efficiency of the drive is reduced due to increased switching and conduction losses, and the performance of the motor is degraded, especially in high speed operation.